Wind turbines typically comprise an electrical backup system, which is provided to increase safety and put the turbine into an idle mode during an emergency situation, for example during loss of a normal power supply. The backup system takes the form of a backup power supply that is often either powered by batteries or ultra-high capacity capacitors. In the event that a main power supply is lost or is unable to pitch the rotor blades, the backup power supply provides the rotor blade pitch motors with enough energy to pitch the rotor blades to a so-called “feathering position”. Once in the feathering position, the rotor blades act to retard the rotation of the turbine such that eventually the rotor comes to a halt and no further power is generated and the turbine is put in an idle mode.
It is important that the functionality of the backup system is verified regularly. The Guideline for the Certification of Wind Turbines, Edition 2010, as well as the International Standard (IEC 61400-1) for the design requirements of wind turbines require that the back-up and safety system of a wind turbine is tested every week.
Methods for performing a test procedure can be found in European patent application EP2824321 A1. Such methods require the measurement of the discharge current and voltage of the backup power supply during testing. Ensuring an acceptable level of confidence in such measurements is an important consideration when testing backup power supplies, as it provides an indication that the measurements can be trusted, and hence provide an indication that the test results can be trusted.